Why Leave a Gap in Crawlspace Insulation?

I read your article “Closed Crawlspaces Do Double Duty,” from the 2005 Special Issue (p. 32), with great interest. The crawlspace in our new home is built over a 4-inch cement floor over plastic and 4 inches of bluestone. The slab is about 3 inches below grade at some points and level with grade at others. This winter it never got below 49ºF in the crawlspace, with temps as low as the ’teens outside. I’d like to insulate the crawlspace walls with 2-inch foil-backed rigid foam insulation. Does the foil face the block wall or the inside of the crawlspace? Do I need to keep a gap at the bottom of the foam, or can it be extended to the floor? Dave Jordan Arlington, Virginia I’d recommend using a product called Dow Thermax, which usually has a foil facing on both sides. (Some versions have a white facing on one side, in which case I’d put the white to the block and have the foil face the crawlspace, for no other reason than to take advantage of the potential radiant barrier effect.) Thermax has passed more stringent fire safety tests than any ...